Californium-252 neutron source and method of making same

ABSTRACT

A RADIATION SOURCE HAS BEEN PROVIDED WHICH IS FREE OF RADIOACTIVE SURFACE CONTAMINATION COMPRISING A SOLID QUARTZ BEAD CONTAINING A SPONTANEOUSLY FISSIONABLE ELEMENT DISTRIBUTED SUBSTANTIALLY EVENLY WITHIN THE CONFINES OF AN OUTER SHELL. A METHOD HAS BEEN PROVIDED FOR PREPARING THE SOURCE COMPRISING SORBING A SPONTANEOUSLY FISSIONABLE ELEMENT ON QUARTZ POWDER IN A QUARTZ CAPILLARY AND SUBSEQUENTLY FUSING THE POWDER AND CAPILLARY TO CONTAIN THE NEUTRON-EMITTING MATERIAL.

Feb. 8, 1972 R BAYBARZ EI'AL 3,640,888

GALIFORNIUM-252 NEUTRON SOURCE AND METHOD OF MAKING SAME Filed Dec. 11.1969 V Fig. 2

INVENTORS.

Russell D. Baybarz By Joseph R. Peterson ATTORNEY.

United States Patent 3,640,888 CALIFORNIUM-252 NEUTRON SOURCE AND METHODOF MAKING SAME Russell D. Baybarz and Joseph R. Peterson, Knoxville,

Tenn., assignors to the United States of America as represented by theUnited States Atomic Energy Commission Filed Dec. 11, 1969, Ser. No.884,089 Int. Cl. C09k 3/00 U.S. Cl. 252-301.1 R 3 Claims ABSTRACT OF THEDISCLOSURE A radiation source has been provided which is free ofradioactive surface contamination comprising a solid quartz beadcontaining a spontaneously fissionable element distributed substantiallyevenly within the confines of an outer glass shell. A method has beenprovided for preparing the source comprising sorbing a spontaneouslyfissionable element on quartz powder in a quartz capillary andsubsequently fusing the powder and capillary to contain theneutron-emitting material.

BACKGROUND OF THE INVENTION The present invention was made during thecourse of, or under, a contract with the U.S. Atomic Energy Commission.

The present invention relates to radiation sources and more specificallyto a glass encapsulated radiation source and method for making the same.

In the art of preparing radiation sources, it is well known to suspendcertain radioactive particles in vitreous fused materials. Generally,the methods for production of such sources take the form of mixing thesource isotopes into a glass forming matrix, subsequently fusing thematrix material at elevated glass making temperatures to bond theisotope in and about the formed glass body. For further informationconcerning these prior methods reference is made to U.S. Pat. 3,114,716to Quinby, issued Dec. 17, 1963 and U.S. Pat. 3,147,225 to Ryan, issuedSept. 1, 1964.

Because of its high probability for spontaneous fission, caliform'um-252is a unique neutron source '(2.3 10 n./sec./ gram). It has outstandingpotential in providing compact, portable and reproducible neutronsources with intensities ranging from a few neutrons per second up togreater than 10 neutrons per second. Of utmost neces sity in any sourcepreparation involving 0f is complete encapsulation, resulting in anexternal surface completely free of radioactive contamination. The abovedescribed prior art methods of glass entrapped sources all suffer fromthe disadvantage of a certain amount of surface contamination. It hasbeen the practice in the art to wash the surface of glass formed sourceswith an acid wash to remove radiocative particles which cling to theouter surface. Since californium-252 is so highly radioactive, it isimperative that there be no surface contamination at all in order toprovide a safe source which does not require a secondary or doubleencapsulation to eliminate external contamination.

'Further, in the art of fabricating Cf sources it has been the practiceto electroplate the Cf onto a platinum needle, to form a mixed oxidepellet of 0f, and to incorporate Cf into a rare earth sol which issubsequently cast into gel microspheres and fired to a mixed oxide.

These techniques suffer from lengthy preparation times, dilution of theCf, multiple mechanical manipulation, and the necessity for ultimatedouble encapsulation of the source to eliminate external contamination.

"ice

Further, it has been found that it is very difficult to control the sizeof the source prepared by the prior art methods as opposed to thepresent method.

SUMMARY OF THE INVENTION It is, therefore, an object of the presentinvention to provide a radiation source embodying a bead comprisingvitreous material with a spontaneously fissionable element distributedtherein.

It is another object of this invention to provide a neutron source asset forth above having a non-contaminating surface formed by an outershell of vitreous material.

Yet another object of this invention is to provide a method for thepreparation of a radiation source as set forth in the above objects.

Still another object of this invention is to provide a method for thepreparation of a radiation source wherein an open-ended impermeablevitreous tube is packed with a porous powder of a refractory materialconsisting primarily of silicon dioxide having a water-immiscibleorganic liquid ion exchange material sorbed thereon, an aqueous solutioncontaining cations of a spontaneously fissionable material passedthrough the thus-treated powder pack and the powder pack is subsequentlydried and heated in air to oxidize and volatilize the organic materialto provide a non-volatile oxide of the fissionable material; and thenfusing the tip and the remaining powder filled portion of the tube toform a vitreous body sealably containing the radioactive oxide. I

Other objects and many of the attendant advantages of the presentinvention will be apparent from the following detailed description ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross sectional view of aradioative source article of the invention;

FIG. 2 is a pictorial view of a capillary tube contain- I ing refractoryquartz powder and the fissionable source material prior to heating;

FIG. 3 shows the tube of FIG, 2 subsequent to heating and forming of asphere encapsulating the source material; and

FIG. 4 shows the spherical source taken from the capillary end and aclosable secondary container for the source.

TECHNICAL DISCLOSURE Referring to FIG. 1, an enlarged cross sectionalview of the present radioactive source article 5 is shown. The articleis preferably a small sphere or spheroid in shape although the shapeneed not be restricted to a sphere-like shape. The spherical shape andsize of the preferred source article is a very close approach to a truepoint source which is significant in many applications where smallportable, maintenance free, efiicient neutron sources are desired. Sucha source may be very useful in cancer therapy, mineral exploration,neutron radiography, oil well logging, small-scale radioisotopeproduction, moisture gauges, on-line analysis of flow process streams,etc.

The sphere or bead is a vitreous material with spontaneously fissionableelements 7, such as Cf, evenly distributed within a sealing outer layercontainment 9 of vitreous material.

The fusion of the quartz capillary into a ball and melting the ball upthe length of the packed bed provide the outer spherical shell of quartz9 which prevents surface exposure of the fissionable material containedwithin. The core 11 of the source article is composed of a matrix ofvitreous material having dispersed therein, in oxide form, radioactiveisotopes of the source material 7. These radioactive isotopes are heldWithin the core matrix by physical forces exerted on the particles bythe vitreous core material, thereby further reducing the extent ofcontamination in the event of accidental fracturing or breaking of thesource articles.

In the process of making the source articles of this invention thefollowing steps are employed: as shown in FIG. 2, an open-endedimpermeable quartz capillary 15 is packed with a porous dry powder,fusible refractory material 17 consisting primarily of silicon dioxide,such as powdered quartz. The powder is first contacted with a waterimmiscible organic liquid ion exchange material such asZ-ethylhexylphenylphosphoric acid or di(2-ethylhexyl) phosphoric acid byadsorbing the liquid onto the porous powder which is then packed to agiven depth depending on the final source size desired. An aqueoussolution containing cations of Cf is caused to flow downward through thepacked capillary whereby more than 99.9 percent of the Cf is sorbed ontothe packing material. Dilute acid is passed through the capillary towash nonadsorbed activity from the walls. The capillary is then dried atan elevated temperature of approximately 125 C. and then further heatedto a temperature ranging from 350 C.400 C. to decompose the organicphase and to convert the Cf to a non-volatile oxide. By using an organicion exchange medium the bulk of the exchange medium is vaporized orburned and the volume of the packing material is not affected byentrapped particles of the exchange material.

Once the fissionable material is sorbed on the powder pack and theorganic material is decomposed by heating, a small torch is then used tofirst melt the lower tip, sealing the bottom end of the capillary andprogressively fus ing the remainder of the packed portion of thecapillary and the fusible powder material into a sphere to completelyencapsulate the Cf, as shown in FIG. 3. This 5 step is performed withina temperature range of approximately 1600-1800 C., well below theboiling point of quartz, in order to fuse the powder pack therebybonding the Cf within the fused powder, and to prevent surfacecontamination by the diffusion of the source particles to the surface ofthe sphere. The sphere can then be cut off and stored in a suitablesecondary container 19, as shown in FIG. 4.

The invention is further illustrated by the following specific example.

EXAMPLE A quartz loading tube about 8 cm. long having a tapered portionalong about 5 cm. of its length and an internal diameter of about 0.5mm. was packed with a bed of organic impregnated quartz powder to adepth of approximately 2 cm. The organic impregnated quartz powder wasprepared by contacting approximately 5 grams of 200-280 mesh quartzpowder with 1 milliliter of an acetone solution containingdi(2-ethylhexyl)phosphoric acid followed by vacuum evaporation of theacetone to obtain an organic concentration in the powdered quartz ofabout 200 mg. of sorber per gram of quartz powder. About 2 millilitersof 0.05 molar HCl containing up to 65 millicuries of Cf was passedthrough the packed column at a flow rate of about 1 milliliter per 10minutes. The column was then washed by passing 1 ml. of 0.05 molarhydrochloric acid through the column. The column was then heated to atemperature of 125 C. for about 10 minutes to drive 01f the water andthen heated to a temperature of 400 C. for about minutes to decomposethe organic material. The column was then fused with a propane-oxygentorch to fuse the quartz powder and tube, allowing the ball of moltenquartz to roll up the length of the column. The fused quartz ball wasthen cut off from the remaining tube and checked for surfacecontamination.

Although the method completely encloses the californium, surfacecontamination from handling in the contaminated atmospheres of a hotcell may inadvertently contaminate the surface of the source. This maybe readily removed by appropriate contact with decontamination solution.

Depending on the desired source configuration, the head can be reshapedby appropriate glass forming techniques to form any shape desired.

The immediate advantages of this method are: (1) source fabricationtimes are short requiring approximately two hours; (2) the necessarymechanical manipulations are suited for hot cell application; (3) theresulting source can be in a nearly true point source configuration; and(4) no further encapsulation steps are necessary although advisable,thus minimizing source bulkiness.

It will be seen that a novel source article and method of preparing thesame have been provided which has a surface area free of radiationparticle contamination and is easier and quicker to prepare than byother methods provided in the art. Obviously many modifications may bemade within the teachings as set forth above; therefore, it is intendedthat the invention be limited only in accordance with the followingclaims.

What is claimed is:

1. A neutron source comprising: an oxide of califormum-252 essentiallyevenly dispersed in a vitreous quartz matrix and an outer shell ofvitreous quartz integral with said inner core, said outer shell beingdepleted of califor- V mum-252.

2. A method of preparing a neutron source, comprising the steps of:

(a) sorbing an organic cation-exchange material contained in an acetonesolution onto powdered quartz, said exchange material selected from thegroup consisting of 2-ethylhexylphenylphosphoric acid or di(2-ethylhexyl)phosphoric acid;

(b) evaporating the acetone to obtain an organic concentration in thepowdered quartz of about 200 mg. of sorber per gram of quartz powder;

(0) packing said powdered quartz treated according to step (a) in anopen ended impermeable quartz capillary to a predetermined depth so asto form a packed column;

(d) passing an aqueous solution containing HCl and a predeterminedamount of californium-ZSZ through said packed column at a rate so as toallow even sorption of substantially all of the californium-ZSZ ontosaid packed column;

(e) heating said capillary and packed column in air to a temperatureranging from 350 C.400 C. for approximately 30 minutes to remove thewater, decompose the organic phase and convert the californium- 252 to anon-volatile oxide; and

(f) fusing said packed column at a temperature ranging from 1600 C.1800C. to form a solid vitreous spheroid containing said californium-252 ina vitreous quartz matrix within the confines of an outer vitreous quartzshell depleted of californium-252.

3. The method of claim 2 wherein said organic cationexchange material isdi(2-ethylhexyl)phosphoric acid.

References Cited UNITED STATES PATENTS 3,364,148 l/1968 Kivel et al25230l.1 3,329,817 7/1967 Walz 252-8011 X 3,442,762 5/1969 Denton252301.1 X 3,334,050 8/1967 Grotenhuis et al. 252301.1 3,457,181 7/1969Haskins 252-301.1 3,147,225 9/1964 Ryan 252-30l.1 3,114,716 12/1963Quinby 25230l.1

CARL D. QUARFORTH, Primary Examiner R. L. TATE, Assistant Examiner US.Cl. X.R.

